A battery unit disclosed in JP 05-205781 A has been known as a battery unit that is mounted on a vehicle and provided with a switching means. This battery unit opens the switching means to disconnect a vehicle load from a battery to prevent overdischarge of the battery, when a voltage drop of the battery continues for a certain period of time in accordance with the increase in the vehicle load.
Such a battery unit generally includes a vehicle power generator, which is connected in parallel with the load, for charging the battery. By opening the switching means, the battery is prevented from being in an overcharge state.